1. Field of the Invention
The present invention relates generally to telephone accessories and more particularly to a telephone call restricting apparatus for preventing unapproved toll calls.
2. Description of the Prior Art
Unauthorized telephone usage is a universal problem, particularly when the telephone is used to place toll calls. Although numerous telephone accessories have been devised which address the problem, a need still exists for an effective call restriction apparatus which is neither overly restrictive nor easy to circumvent. In other words, an apparatus which permits toll free long distance calls such as those to an "(800)" or area code plus "555" number, but which prevents operator assisted and local toll calls.
To better understand the problem, a review of the various telephone number categories comprising the direct distance dialing system is in order. The first category includes the one to four pre-prefix digits often required in a private branch exchange (PBX) system. For example, many such systems require that a "9" be dialed in order to get an outside line.
The second category includes the one to three prefix digits required in certain parts of the country. For example, in some locations a "1" must be dialed before a long distance call is dialed.
The third category includes a three digit number required to place a long distance call. This three digit area code number must be dialed when wanting a toll number outside the dialing area code. An area code is easily distinguished from the fourth category three digits representing the central office category, since the second digit of an area code must be either a "0" or a "1". The second digit of a central office code, like the first digit of the area code category, is limited to the digits "2" through "9".
The fifth and final category of numbers includes four digits which comprise the local number.
Numerous prior art devices exist which place restrictions on the use of a particular telephone device. These devices include simple dial locks and sophisticated computer systems such as the one disclosed in the U.S. Pat. No. 3,931,476 of Gordon H. Matthews and which prevents calls from being completed unless a computer-verified user identification number is first dialed.
Another class of restriction devices monitor the telephone number as it is being dialed and prevent completion of those calls which meet certain conditions. For example, in U.S. Pat. No. 3,681,536, Pask et al disclose a device for preventing the dialing of those long distance calls in which a long distance prefix digit is required by aborting all calls in which the first digit dialed is a "1".
Recognizing that the second digit of an area code is either a "0" or a "1", Piacente et al suggested device in U.S. Pat. No. 3,899,640 for aborting those calls in which a "0" or a "1" is the second digit dialed.
Further realizing that an operator assisted call may be a toll call, Klaiber et al disclosed a device in U.S. Pat. No. 3,680,762 for additionally aborting calls in which the first digit dialed is a "0".
A somewhat different approach is taken by Chan et al who disclosed a device in U.S. Pat. No. 3,980,836 for not only inhibiting calls in which the first digit dialed in a "0" but also those calls in which the number of digits dialed is in excess of that required for a local call.
A much more elaborate device is disclosed by Downs et al who disclosed a device in U.S. Pat. No. 3,851,109. In general, the device may be programmed to respond to the dialing of a preprogrammed disallowed digit after a preprogrammed number of digits have been dialed and operate to set a latch. Next, if an override signal is not generated, following the dialing of a predetermined number of digits the call is aborted.
The device further includes programmable code detectors which respond to preselect patterns of digits and operate to generate the override signal. Examples of the preselected patterns include the area code "(213)" and the information number "1-(XXX)-555-1212".
Finally, the device includes a timer to detect further dialing following the termination of a previous call.
Concerned with preventing local toll calls, Rolen et al disclose in U.S. Pat. No. 3,985,972 a device for comparing the signals representing the central office digits of a telephone number to a prestored table of central office toll-free numbers for aborting dialed unapproved toll calls. To further prevent operator assisted toll calls a circuit is included for aborting those calls in which the first dialed digit is a "0".
A problem with the device of Rolin et al is that it fails to recognize calls which include a prefix digit or those in a PBX environment in which a pre-prefix digit is employed.
Another problem with prior art devices for preventing long distance calls is that they also prevent toll-free long distance calls such as those including an "(800)" area code or an area code plus "555" central office code.